The present invention relates generally to step motors and more particularly to input control circuit for a step motor wherein command pulses (representing the number of motor steps to be taken) can be received from an undedicated source .
As is well-known in the art, a step motor operates by movement in rotational increments or steps. The number of steps to be taken in any particular application is inputted to the control circuitry of the step motor as shown, for example, in U.S. Pat. Nos. 4,119,901 and 4,119,902. The input signals are commonly referred to as command pulses, and each command pulse represents one step of motor movement. The command pulses can be generated by any one of numerous means, such as a microprocessor programmed to run a series of step motors.
Step motors have two modes of operation, low speed and high speed. High speed operation is commonly referred to as "slewing". In each mode, the step motor initiates and terminates movement at an "error free start stop" speed, commonly referred to as "EFSS".
More particularly, the step motor begins stepping as command pulses are received. In response to the command pulses, the drive circuitry of the step motor generates motor pulses on a one-for-one basis, and these motor pulses energize the windings of the step motor and effect operation thereof. The step motor reaches "EFSS" speed quickly (usually by the third motor pulse) and then further accelerates. After a time, deceleration is initiated and continues until "EFSS" speed is again reached. The step motor then stops. Slewing is achieved only when the number of steps to be taken is sufficient to permit acceleration to full or slew speed.
As shown and described in U.S. Pat. Nos. 4,119,901 and 4,119,902, the rate of acceleration is determined by the difference between the number of command pulses received and the number of motor pulses issued. The digital representation of this difference is converted to an analog voltage, which is applied to a voltage-controlled oscillator. The magnitude of the analog voltage sets the motor pulse issuance rate.
Thus, the acceleration/deceleration pattern is controlled by the rate of inputting command pulses. When slew speed is reached, the command pulse source is issuing command pulses at a fixed rate, equivalent to the motor pulse rate. The analog voltage received by the voltage-controlled oscillator is therefore constant, and slew speed is properly and accurately maintained.
However, the command pulse source must remain dedicated or married to the input circuitry of the step motor throughout the operation. In other words, the step motor is incapable of proper operation unless the command pulse source remains "on line" throughout the operation sequence, i.e., from receipt of the first command pulse through initiation of deceleration. As such, the input circuitry of the step motor cannot be preloaded, i.e., the command pulse cannot be inputted prior to operation of the step motor.
This incapability on the part of the step motor results in a virtual forfeiture of the capability offered by a microprocessor when used as a command pulse source. The microprocessor has more than sufficient capacity to act as the command pulse source for numerous step motors. However, since the microprocessor must remain dedicated to each step motor during operation, its capacity cannot be utilized.
Proper step motor operation also requires the constant, accurate monitoring of command pulses and motor pulses. In the past, the hardware necessary to properly monitor the command pulses and motor pulses has been expensive and designed to isolate the two pulse series, such that miscounts do not occur. Generally, there is up-counter device for each pulse series, interconnected to an adder which supplies a signal digitally representing the difference therebetween.